Method and apparatus for stabilizing plaster

ABSTRACT

The invention provides a process for stabilizing plaster and an apparatus for implementing the moistening part of the process. The process comprises the following steps: a) providing a heated HH-plaster preferably at temperatures over 100° C.; b) feeding the hot plaster in a moistening device having walls heated to at least 100° C.; c) injecting water and/or steam into the moistening device in conditions that such not yet moistened surfaces of the plaster are exposed to the injected water and/or steam; d) maintaining an atmosphere in the moistening device at a level of the dew point in the range of 75 to 99° C.; e) feeding the moistened blend into a curing device; f) maintaining an atmosphere in the curing device above 75° C. for at least 3 minutes; g) feeding the moistened and cured blend into a drying device; and h) drying said moistened and cured blend. 
     The invention also provides an apparatus for moistening β-hemihydrate plaster comprising a rotating drum with lifting blades inside where all walls in contact with the product are externally heated at a temperature above 100° C.

TECHNICAL FIELD

This invention deals with the post-treatment of a calcined gypsumplaster, said treatment being known under the terms forced ageing orstabilization.

BACKGROUND ART

Gypsum is calcium-sulphate dihydrate (DH) of the formula CaSO₄.2H₂O.Vast deposits of natural gypsum provide gypsum rock or gypsum sand.Synthetic gypsum originates from the phosphoric acid production and moreand more from Flue Gas Desulphurization (FGD).

Plaster, in this context and in the generally accepted terminology ofthe art, is partially dehydrated gypsum of the formula CaSO₄.xH₂O wherex=0 to 0.5, with the potential to re-crystallize to a solid structurewhen mixed with an appropriate amount of water.

Calcining means the thermal treatment of a DH in order to remove a partof the combined water.

Hemihydrate (HH) or semihydrate (SH) is the metastable hydrate of theformula. CaSO₄.½H₂O.

Anhydrite III (AIII) is a dehydrated HH with the potential of reversiblyabsorbing water or even vapor. The reversible uptake of water liberatesconsiderable reaction heat.

Anhydrite II (AII) is the completely dehydrated product. It is formed athigher temperatures and is not welcome in stucco plasters and therefore,in calcining for industrial plasters, conditions to create AII areavoided as far as possible.

HH and AIII are the products resulting from the first steps ofcalcining. Whether AIII or HH is first formed depends on the calciningtemperature and the vapor pressure in the calcining ambience.

Generally plasters are calcined under dry conditions meaning in hot airor in an indirectly heated calcining vessel. Under those conditions thesize and shape of the DH particle of origin remains essentially thesame. Thus the resulting plaster is porous. It is ordinarily calledstucco or plaster of Paris. The accepted technical term is β-Hemihydrate(β-HH).

The possible use of plasters as a binder results from its ability tobuild up a completely new crystalline structure out of an aqueousslurry. This is due, in the first place, through the very big differencein the solubilities of HH and DH (about 8 g/l vs. 2.7 g/l). Thus, a HHcreates a tremendous over-saturation with regard to DH. Theover-saturation leads to the formation of germs and quickre-crystallization sets in.

Normally, salts increase their solubility with the temperature.Ca-Sulphate behaves rather irregular in decreasing its solubility. Thesolubility curves of HH and DH cross each other at about 100° C. In thetemperature range between 85 and 100° C. the differences in solubilityare so small that setting virtually does not start at all. At 75° C. thereaction rate is still very low.

Due to the rough thermal treatment the physical microstructure of β-HHis stressed and quite unstable. Thus, one observes that, in contact withliquid water, a β-HH will partially disintegrate into very smallparticles. However, by absorbing humidity, the stress is lowered and thedisintegration phenomenon fades. Simultaneously the speed of dissolutionin water diminishes. The phenomenon is called “ageing”. The term isquite misleading because it is more an effect of the ambient conditions(humidity, temperature) than of time.

Because of ageing, β-HH has the tendency to change its rheology andsetting kinetics over the time dramatically. The drift in rheology iscaused by the diminishing tendency of β-HH to disintegrate, as explainedabove, in very fine particles. The drift in kinetics has to do with the“healing” of crystalline defects (spots of heightened activity) in thecalcined product.

The starting point of the drifting properties depends largely upon theorigin of the plaster, the granulometry and the calcining conditions.There is a widely accepted consensus that the adsorption of water is themain promoter of ageing. AIII can take up as much humidity as to becomeHH. Then, surprisingly, the uptake of water continues until about 8%combined water, which is significantly above the theoretical value ofHH, without forming DH.

Ageing is a problem in construction/wall plasters where the conditionsof storage and the delay between calcining and application can vary in awide range. In plasterboard production ageing is a problem as well,albeit to a lesser extent.

Plasters, which have reached their virtual final state of ageing, offertwo main advantages:

a) constancy and reliability;

b) control of the granulometry and, thus, of the rheology.

This has the effect of for example, without being limited thereto:

-   -   less overall variation in product qualities;    -   less water to dry out in plasterboard manufacture; and    -   less retarder in construction/wall plasters;    -   less ultrafines in gypsum fiber boards, resulting in an easier        dewatering.

The art knows since long time how to age a plaster forcedly. The basicidea is quite simple: give all, or even more, of the water at once thatis needed to quench the “thirst” of the plaster. The process has beencalled “stabilization” in the prior art.

Note that forced ageing or stabilization in the sense it is used in thepresent invention is not “aridisation” which is essentially calcining inthe presence of deliquescent substances (see e.g. U.S. Pat. No.1,370,581).

U.S. Pat. No. 1,713,879 is apparently the first publication dealing withstabilizing. It discloses the mixing with water and/or steam with acalcined plaster. The purpose is to reduce the water demand and thetemperature rise during setting. The figures are: 12 to 15 pounds ofwater/minute for one ton of plaster over a period of 5 to 6 min(equivalent to 5 to 9% water in total). The plaster is preferably a“single boil plaster” (i.e. essentially HH without AIII). It is a batchtype operation. There is no mention of temperatures or specific featuresof the equipment used. A variant is the introduction of water by meansof a carrier like diatomaceous earth. The process is called (forced)ageing and not yet stabilizing.

DE-A-553519 discloses a process of treating calcined plaster with waterand/or steam in order to render the plaster less sticky and less waterdemanding. The amount of water absorbed is 0.5 to 7%. It uses thereaction heat of AIII in order to heighten the temperature. Thetemperature at the end is between 80 and 130° C. and should not exceed140° C. The patent does not disclose limits for the curing time butgives an example of half an hour of treatment of plaster with theexhaust gases of a rotary kiln. The temperature of the plasterdischarged is 95° C. There is no mention about drying but. There is amention that the treatment can be done in rotary apparatus, which allowsan intimate contact of the steam with the product.

U.S. Pat. No. 1,999,158 mentions and relies apparently upon U.S. Pat.No. 1,370,581 (aridisation). The field of application is wall plasters.The claimed improvement lies in the superfine-grinding in order toincrease the plasticity and to reduce the change in setting time overelapsed storage time of the powder. Plasticity is defined by the USconsistency of 65 to 75. The fineness of the ground plaster is describedas having a large part smaller than 10 μm. (Note that the termstabilized plaster is first used).

U.S. Pat. No. 2,177,668 deals with forced ageing which is in this caseessentially the reversion of AIII to HH by the treatment of the calcinedplaster with huge amounts of air of about ambient RH (60% RH) and atemperature just below the theoretical stability temperature of DH at42° C.

U.S. Pat. No. 3,415,910 discloses quenching of a hot plaster with waterwhilst maintaining a temperature high enough to avoid the formation ofDH (between 82 and 100° C.) and subsequent heating above 102° C. (dryingup to 157° C.). The moisture content at the highest was 3%. The dryingwas done up to the point where the theoretical value of combined waterfor HH was attained. The preferred (and exclusively described method)was using a kettle as calciner and utilizing the same kettle as thedevice for the treatment and subsequent drying step. The plasterobtained and claimed is characterized by: (i) density at 20° C.=2.60g/ml (<10% below 1.6 and <10% above 2.68 g/ml) and (ii) stacking orderindex above 8. The patent describes the role of disintegration on waterdemand and the rheological properties.

GB-A-1233436 is essentially equivalent to U.S. Pat. No. 3,415,910.However some slight differences and additional information aredisclosed, suggesting that the process has been further developed. Forexample the maximum moisture has risen to 3.5%, the admissible calciningtemperature is now 160° C. Treatment temperature in laboratory could beas low as room temperature. A preferred treatment temperature inindustrial application is between 82 and 93° C. A preferred dryingtemperature is above 115° C. Graphs demonstrate the effect of freemoisture and of curing time on the US consistency suggesting that 3% at3 min are the lower limits of operation.

U.S. Pat. No. 3,527,447 is an improvement over U.S. Pat. No. 3,415,910.It discloses the drying step carried out in a separate device undersub-atmospheric pressure. In order to maintain the required temperaturerange an additional energy input by means of microwaves is suggested.

U.S. Pat. No. 4,117,070 (and related U.S. Pat. No. 4,153,373 andFR-A-2383893) proceeds to a continuous method for stabilizing withoutdrying as part of a plasterboard production process. In a specificembodiment 50 to 75% of the board line feed are treated with 1 to 8% offree water cured for about one minute and this feed is then recombinedand mixed with the remaining portion of the feed which is cured anotherthree minutes. The total moisture after recombination is 3-4%. Isdisclosed a fluidized and agitated vessel as a wetting vessel.

EP-A-0008947 deals with the inconvenience of storing a longer timewetted plaster. It introduces the notion of “set suddenness” which isthe maximum temperature rise during setting. High set suddenness isdisclosed as essential for the development of an adequate mechanicalresistance and is substantially reduced by the stabilization procedure.The remedy for this drawback is grinding the treated (dried or not)plaster to a fineness 3 to 4 times the original (measured in Blaine).

GB-A-2053178 discloses the simultaneous grinding and wetting in an“Entoleter” mill or the like. Curing happens after the size reduction.The set suddenness of EP-A-0008947 is attained by this procedure aswell.

Those patents differentiate between forced ageing (i.e.quenching/moistening) and stabilizing (i.e. quenching/moistening andcuring, and optionally drying). They claim different approaches toobtain an aged or a stabilized plaster and specify appropriategranulometries for the use in plasterboard production.

Every stabilizing method includes the steps of moistening and curing.Moistening is the trickiest part, but curing has some problems as well.Two main concerns are: (1) unintended rehydration, which creates DH,acting as crystallization seeds in plaster slurries and (2) built-ups orscaling in the equipment.

The formation of DH occurs if liquid water and plaster are in contactover a certain time under thermodynamic conditions allowing the reactionthat is at lower temperatures. It is obvious that the moistening of abinder like plaster leads inevitably to the formation of lumps and thatevery surface in contact with the moistened product and/or themoistening liquid has a tendency to build up crusts of potentiallyhardened matter. The problems are more pronounced in the moistening partof the devices because moistening includes mixing which produces dustand includes the presence of water which can lead to condensation.

What has been disclosed in the related art with regard of solving theproblems mentioned above is not satisfying. In U.S. Pat. No. 3,415,910,the use of a kettle with the need to cool down and reheat the wholeequipment is time and energy costly. U.S. Pat. No. 4,153,373 describesas a wetting apparatus a fluidized and agitated vessel used in theprocess of treating plaster for plasterboard production. Here theformation of traces of DH does no harm because the plaster is to beaccelerated anyway in the plasterboard line. GB-A-2053178 combineswetting and grinding in one step also in the context of plasterboardproduction. Scaling is here avoided by shear forces but the DH issueremains unsolved.

SUMMARY

The present invention aims at solving the problems and provides aprocess for stabilizing a β-Hemihydrate plaster (β-HH) and an apparatusfor implementing the process. It has been found that the scaling and DHproblems can be solved by maintaining conditions from moistening untildrying where hydration of the HH cannot take place. In the invention thehygro-thermal conditions are controlled in the space of treatment andthe temperatures of parts in contact with the product are controlled.

Additional mechanical measures might be taken to prevent build-ups ofproduct on the parts of the equipment involved.

Thus, the invention provides a process and apparatus as defined in theclaims.

BRIEF DESCRIPTION OF DRAWINGS

The embodiments of the invention are disclosed in reference to theaccompanying drawings, in which:

FIG. 1 is a general scheme of the overall process of the invention;

FIGS. 2 a and 2 b show a preferred embodiment of a moistening device;

FIG. 3 shows a preferred embodiment of a combined moistening/curingdevice;

FIG. 4 shows a preferred embodiment of a combinedmoistening/curing/drying device.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PREFERRED EMBODIMENT

As indicated above, the invention provides a process of stabilizing acalcined β-hemihydrate plaster by moistening and curing and optionallydrying comprising the steps of:

a) providing a heated HH-plaster preferably at temperatures over 100°C.;

b) feeding the hot plaster in a moistening device having walls heated toat least 100° C.;

c) injecting water and/or steam into the moistening device in conditionsthat such not yet moistened surfaces of the plaster are exposed to theinjected water and/or steam;

d) maintaining an atmosphere in the moistening device at a level of thedew point in the range of 75 to 99° C.; feeding the moistened blend intoa curing device

e) maintaining an atmosphere in the curing device above 75° C.,preferably between 75 and 99° C. , for at least 3 minutes, preferablybetween 4 and 15 minutes;

f) feeding the moistened and cured blend into a drying device; and

g) drying said moistened and cured blend.

Optionally, the process can comprise the following steps:

h) grinding the dried product; and/or

i) cooling the dried product, where the cooling can take place before orafter the grinding step.

In the above process, it is preferred to provide a metering of thecombined water and/or steam amount so as to get 3 to 12%, based on theweight of the HH, of free moisture in the moistened blend.

In the above process, the atmosphere of the moistening and/or curing canbe controlled by (1) controlling the flow of external air trough themoistening and/or curing device and/or by (2) controlling the heating ofthe walls of the moistening and/or curing device.

The process described above can be carried out batch-wise or in acontinuous way, the latter being the preferred one. In this case some ofthe mentioned steps (and corresponding devices) might be zones of asingle device. In one preferred embodiment moistening and curing arecombined in the same apparatus.

Hence, steps d), e) and f) are preferably combined into a single step of“moistening and curing”. Also, steps h) and i) are preferably combined.

FIG. 1, which is a general scheme of how the treatment works, depictseach single step and the potential combining of several steps, with thepreferred embodiments of the invention.

As far as steps a) and b) are concerned, it should be noted that theprocess has to be run at elevated temperature in order to avoidrehydration. In a stabilizing step, implemented in a plaster plant, theplaster comes out of the calcining device generally at temperaturesranging from 155 to 180° C. During the first steps of conveying it coolsdown generally to 120 to 140° C. In the case of the higher domain oftemperature the need of cooling down the plaster occurs. Since theplaster has to be moistened cooling can be done by direct injection ofwater which will cool by evaporation and by heat exchange. If thecooling is done in the same device as the moistening an additionaladvantage is the generation of sufficient vapor to create high humidityin the atmosphere of the moistening device raising the dew point to thedesired temperature of 75 to 99° C.

As far as steps c) and d) are concerned, it should be noted that in thestabilization process liquid water should be present. When water aloneis injected it is clear that water will be present; when steam is usedthen condensation at cooler parts occurs. If locally the offer of wateris higher than the demand by the absorptive capacity of the plasterpresent, the plaster particles will stick together and will form more orless persistent lumps. If the water adheres on a part of the equipmentthe plaster will stick on this part. Hence, the water and/or steam ispreferably metered so that condensation, if any takes place, whencombined with added liquid water if any, will produce the requiredamount of water.

The range of percentage of added water and the corresponding neededcuring time is known. The related art shows that there is a certainevolution in the behavior with increasing water addition and curingtime. At higher figures the change levels out to “saturation”. In orderto avoid changing properties it is preferred to work at this“saturation” level. It is reached above 3% free moisture and above 3minutes curing time. At high water addition the behavior of the plasterwill get closer to the normally gauged plaster. An upper limit of freemoisture is thus preferably 12%.

As far as step e) is concerned, it should be noted that maintaining atabout the dew point is important because this is the temperature ofequilibrium between temperature and evaporation rate. Otherwise, thetemperature of the system will always shift rapidly toward the dewpoint. If external air with a low dew point enters the device it willinevitably cool down the product even if the air is much hotter than theproduct.

The atmosphere can be controlled by air introduced and/or by heating thewalls. Heating the walls is already there in order to avoidcondensation. The heat transfer through the walls can contribute to thethermal regime by compensation heat losses by evaporation. In this casethe product temperature can by slightly above the dew point.

As far as step f) is concerned, it should be noted that curing time isalso an important parameter. The needed time depends upon the nature ofthe plaster, the temperature and moisture. The minimum required here is3 minutes. However longer times are preferable because they will tendmore into the saturation domain. Typical curing times are from 4 to 15minutes.

As far as step h) is concerned, it should be noted that the dryingtemperature is not critical in the first place and can go up to about160° C. (see the previously cited prior art documents). However, it hasbeen found that drying at lower temperatures gives more reproducibleresults. A product temperature below 115° C. is preferable. A specialdrying method works at temperatures below 105° C. It has been found thatunder this condition the drying stops at total water content of about7.0% LOI (Loss Of Ignition). This LOI is significantly higher than thetheoretical LOI of 6.2%. (all figures based at 100% purity of theplaster). Surprisingly plasters with this LOI do not, as one couldexpect, recombine the over-stoechiometric water into DH. Even an LOI of8% is admissible. In this respect they are alike plasters having beenaged naturally at severe conditions (60° C. and 90% RH over more than 24h).

As far as step i) is concerned, it should be noted that the grindingneeded to get the required fineness depends largely of the nature of theraw gypsum used and the intended use of the stabilized plaster. Formolding purposes as well as for plasterboard production a fineness ofabout d50=15 to 22 μm is optimal. For filtering processes the naturalparticle size distribution (PSD) of FGD gypsum is very well suited. Forcompression-filtering processes a broad PSD is preferable. In any casethe PSD given by a grinding and/or selecting process is maintained inthe aqueous slurry. A bimodal distribution can be obtained as well,especially starting from FGD gypsum.

As far as step j) is concerned, it should be noted that very often β-HHhas a certain percentage of AIII. In contact with water AIII rehydratesto HH. In ordinary plasters AIII plays useful role in absorbinghumidity. If plaster is stored in bags, moisture diffuses from outside.By absorbing this moisture AIII acts as a buffer and prevents for acertain time the altering of the properties induced by said moisture.Hot plaster stored in a silo will cool down slowly from the walls. Thisprocess can induce condensation near the walls. Sufficient AIII can thenprevent the formation of DH. A stabilized plaster has no AIII. Thus, ifstored in a silo it should be cooled down sufficiently in order to avoidcondensation. Hence, it may be useful to have a determined cooling step.

The stabilized β-HH of the invention is stable with regard todisintegration and setting kinetics. It is substantially free of AIIIand its combined water is above the theoretical value. Any required PSD,adapted on its specific use, can be obtained by grinding sifting and/orblending.

The stabilized β-HH of the invention is useful as a binder and/or fillerin wall plasters, for gypsum fiberboard production according to afiltration process, for plasterboard production, for industrialplasters, for jointing compounds, for high strength gypsum fiberboardproduct according to a press-filtering process, etc.

For every application a specific PSD in the aqueous plaster slurry isoptimal and can be properly attained by suitable grinding of a plastertreated according to the present invention.

For example, one can cite:

-   -   d50 of 30 to 100 μm: as a binder and/or filler in wall plasters.    -   d50 of 20 to 30 μm: for gypsum fibreboard production according        to a filtration process.    -   d50 of 15 to 22 μm: for plasterboard production.    -   d50 of 10 to 20 μm: for industrial plasters and/or for jointing        compounds.    -   bimodal with a first peak at 3 to 10 μm and a second peak at 20        to 60 μm: for high strength gypsum fibreboard production by the        press-filtering process.

Devices for implementing the process of the invention should be able tobe operated under the indicated hygrothermal conditions. For themoistening/curing part e.g. a rotary shaft granulator as used forfertilizer or a device like a glue blender for particle board with steamjacket is suited. An air mix® granulator fed with steam loaded air andheated walls is suited as well. For the drying part almost every drierfor powders is suitable (as long as it does not allow cooling down themoist plaster to conditions allowing rehydration).

FIGS. 2 a and 2 b (side view of 2 a along lines AA) show an embodimentof the invention. The moistening device comprises a drum 1 rotating inheated space 2. The drum is supported on bearings 3 and driven by amotor 4 outside of the heated space so that the complete surface of thedrum, potentially in contact with plaster, is heated. Feed andextraction are done by conveyor screws. The feeding screw 5 is ametering screw. A certain level of product is maintained in the hopper 6(equipped with a level sensor) in order to seal the interior of the drumfrom the outside. The extracting screw 7 runs at a speed suitable forextracting the incoming plaster. The product is collected in thedischarge housing 18 and finally extracted by a dish extractor 19. Themoistening happens through two phase (vapor-water) pulverizing nozzles11. The metering is performed by an appropriate device as for example acombination of flow-meter and control valve 9. The interior of the drumis equipped with (lifting) blades 10 able to uptake the plaster and torelease it successively as shown by the arrows in FIG. 2 b. The plastercascade is sprayed with water and/or vapor. The spraying nozzles areprotected from being covered with plaster by a roof 12 which isconstantly cleaned by the passing blades 10. One feature not shown inthe drawings is that the blades in the area of the roof are fixed onspring steel ribbons. The roof is slightly inclined so that the bladesbecome under tension and spring back when they have passed the roof.Then they hit an obstacle. The shock shakes off potential build-ups. Thedrum is heated from the bottom outside by a row of gas burners 14. Itspower is controlled by the temperature T2 measured on the upper side ofthe drum which controls a valve 13; it may also take into accounttemperature T1 of the plaster in the hopper 6. The temperature of thedischarged product is essentially the dew point of the atmosphere in thedrum. It is measured by the temperature T3 which controls the airflowthrough the flap valve 15 in order to maintain a given producttemperature. The air extracted here is more or less saturated withsteam. Also a temperature T4 will control valve 16 so as to control theflow of air in the moistening zone and accordingly the atmosphere inthis zone. Another flap valve 20 let air from outside dilute the humidair in order to avoid condensation in the filter. A fan 17 provides thenecessary power to entrain air. A controlled gap between the drum 1 anda cover plate 21 provides the inlet for external air. The necessary sizeof the moistening device results of the throughput and the time neededfor the moistening process. Generally a minute is sufficient forspraying (space 22); another two minutes for homogenizing (space 23) isrecommendable. The table below indicates reasonable dimensions for amoistening drum using the times mentioned above:

Capacity [t/h] 10 20 40 diameter [m] 1.0 1.3 1.6 length [m] 3.0 4.0 5.0

Since the curing period runs under the same hygro-thermal condition asthe moistening it is obvious that it is conveniently conducted in thesame equipment extended so as to allow the intended time of residence.

FIGS. 3 a and 3 b (side view of 3 a along lines AA) show anotherembodiment of the invention. Thus FIG. 3 shows a variant if the devicein FIG. 2. The same reference will designate the same part. The curingzone (space 24) is attached to the moistening part but is of largerdiameter. It is separated from it by a barrier 25. Reasonable dimensionsof the moistening zone are:

Capacity [t/h] 10 20 40 diameter [m] 1.4-1.6 1.8-2.0 2.3-2.6 length [m]4.2-3.2 5.3-4.0 6.7-5.0

The drying of the cured plaster can be done in many ways known in theart. A stream drier is well suited but consumes relatively much energyfor its large volumes of air moved through a cyclone and a bag-house.

FIG. 4 shows a further embodiment in which the steps of moistening,curing and drying step are combined. In the first part of the drummoistening and curing takes place in a drum designed as in FIG. 2 butlonger in order to provide curing space. Attached, in prolongation ofthis drum, but separated by a notch-shaped wall 26, is a second part 27of the drum which is more heated from the outside and ventilated withhot air. The path of the hot air is shown by the arrow 28. In the part27 the moisture is dried out at product temperatures not exceeding 110°C. and preferably below 105° C.

In another embodiment of the invention the steps of drying and grindingare combined. Mill-drying as with an Impmill® or an Ultrarotor® and manyothers combines drying and grinding. Such a combination is useful whenthe plaster is to be finely ground.

Another preferred combination is grinding and cooling. If the productleaves the drying step according to the embodiment of FIG. 4, itstemperature is already relatively low. A small percentage of freemoisture may remain. The air forced trough the mill will dry theremaining moisture and cool simultaneously.

The following examples illustrate the invention without limiting itsscope.

For the purpose of testing the process of the invention, a continuousworking pilot installation has been built. The pilot was of the typeshown in FIG. 2 (save that instead of rotating in a heated space thedrum had a double jacket which was gas fired). The pilot was fed with aplaster of FGD origin which had been calcined in an indirectly steamheated rotary calciner. The temperature of the feed at entrance in thepilot was 120° C. in average. The capacity was 150 kg/h for the combinedsteps of moistening, curing and drying. Moistening was set to 4%±1%. Theaverage residence time was set to 16 minutes. In the combined moisteningand drying mode the pilot simulated the apparatus designed in FIG. 4. Ifone considers a free moisture below 2% at the beginning of the dryingprocess it can be assumed that the moistening and curing time togetherwas in the order of 5 to 10 minutes (which is about in the saturationlevel). The control parameters for the product were the temperature andthe moisture. The process could be controlled by the air flow allowed topass the drum in counter current and the external heating temperature.The air flow was regulated by the width of a gap at the feeding side inorder to maintain the product temperature at the required level. Thetemperature of the external heating reached 185° C. Under thoseconditions the product left the drum at a temperature of 100±5° C. and aLOI of 6.5 to 7.5%. In the moistening mode the internal space wastightened in order to avoid significant air exchange. The product at thedischarge had lost a maximum of two percent of the original moisture.The external heating reached 125° C. The temperature of the dischargedproduct was around 85° C.±10° C. In this mode only the occurrence of DHwas of interest. Small samples were quickly dried in an oven at 50° C.and tested for combined water and by Differential Thermal Analysis(DTA).

The following plaster samples were tested, as reported in the tablebelow. All samples are made out of the same source of raw gypsum whichis a FGD gypsum of a German lignite fired power plant and all arecalcined in the same aster plant which uses a indirectly steam firedrotary kiln. Samples 1a, 1b, 2, 3 and 4 are of the conventional type.Samples 5 to 9 are according to the invention.

TABLEAU 1 Sample Type  1a FGD gypsum calcined, not ground, from silo  1bFGD gypsum calcined, not ground, after calciner  2 FGD gypsum calcined,not ground, after 4 month storage in a plastic bag  3 FGD gypsumcalcined ground to d50 = 28 μm, fresh  4 sample 3 stored 24 hrs at 35°C./90 RH (conventional forced ageing)  5 Moistened, cured, dried,cooled, not ground, fresh  6 sample 5 but after 4 month storage  7sample 5 but ground to d50 = 24 μm  8 sample 5 but ground to d50 = 12 μm 9 sample 5 but ground to d50 = 5 μm 10 50% sample 6, 50% sample 8

For the purpose of determining the mechanical stability of a plaster wedefine first:

The slump is the diameter of a slurry cake produced with a Schmidt ring,and slump 1 is the slump of a plaster gauged by hand while slump 2 isthe slump of a plaster gauged with a blender shaft type Braun® MR400,300 W for 20 s. The Water/Plaster (W/P) ratio is maintained at 0.75.

For the purpose of judging the stability we define as reference theslump of a stabilized plaster of a given PSD with the slump of a givenplaster of the same PSD. The stability factor 1 is the ratio of theslumps hand gauged. The stability factor 2 is the ratio of the mixedplasters. All were at a W/P ratio of 0.75.

The PSD of plasters is measured by a Laser Granulometer of the typeMalvern® Mastersizer, with the plaster being dispersed by ultrasonictreatment in alcohol.

For the purpose of determining filterability we blend 100 g of plasterwith 500 g of water (containing enough retarder to allow the operationwithout setting). The suspension is given into a cylinder of 80 mmdiameter. Pressured air of 1 bar pushes the water through the filter.The water released is recorded over the square of the time. In this caselinear curves are obtained. The slope of the curves gives an indicationof the filterability of the plaster. As above, filterability 1 relatesto hand-gauged whereas filterability 2 relates to mixed suspensions. Thehigher the value the faster goes the filtration. A stability factor 3 isdefined as the ratio between the two slopes. It gives an indication howmuch the filterability is affected by the mixing process.

It has to be noticed that the slope measured this way is only a simpleindicator. To translate it into filtering times needed for a givenpercentage of water released one must read the diagram and calculate thesquares of the abscises values.

The following table shows the properties of the various plaster samples.

Combined US con- Slump 1 Slump 2 Stability Stability Filtera- Filtera-Stability Sample water % sistency mm mm factor 1 factor 2 bility 1bility 2 factor 3 1a 5.6 — 150 220 — — — — — 1b 5.4 — 140 130 — — — — —2 5.8 — 130 135 0.65 0.57 35 9 0.28 3 5.8 66 225 180 0.94 0.71  9 5.50.62 4 7.2 — 205 245 1.03 1.04 — — — 5 7.2 195 230 0.98 0.98 — — — 6 7.0— 200 235 1.00 1.00 — — — 7 7.0 56 240 255 1.00 1.00 — — — 8 6.7 62 265290 — — 13 10 0.88 9 7.0 74 210 230 — — — — — 10  7.0 60 — — — — 27 230.85

Sample 2 vs. sample 4 shows the effect of conventional ageing.

The values in the table also shows that the accelerated ageing accordingto the invention yields the essentially the same product as conventionaltime-demanding ageing.

Sample 2 vs. samples 5 and 6 shows clearly the effect of the treatmentof the invention on the not ground material with regard to slumps andstability factors of any kind. The same holds true for the comparison ofsample 3 vs. sample 7. Even at slightly lower PSD the slump values ofsample 7 are higher bigger than of sample 3. The slump 2 is even higherthan slump 1.

The comparison of the samples 7 with sample 8 shows that a fineness ofd50=12 μm gives a better fluidity than the coarser material. Thisphenomenon is as surprising as is the absolute value of slump 2 ofsample 8. Equally surprising are the absolute values of slump 1 and 2 ofsample 9. With a non treated material one can expect less than 150 mm.The filterability of sample 8 is better than that of sample 3 with amuch coarser PSD.

Surprisingly sample 10 which is a blend of 50% sample 5 and 50% sample 8gives filterabilities of about the average of both. Translated infiltering time the ratio sample 7/sample 5 is 6.3 while the ratio sample8/sample 10 is 2.2.

Compared with the standard product of non-treated plaster, treatedplasters as in sample 7 to 9 have considerable advantages in manyapplications where a low water demand or a high fluidity at a given W/Pratio is required. This is the case for plasterboard production orprefabricated (molded) products of any kind.

For industrial plasters, in addition to low water demand, constancy isprimordial. Treated plasters according to the invention offer thisconstancy over different production lots and over a long storage periodbecause they are treated at the saturation level.

The resistance to mechanical destruction during mixing of treatedplasters is unmatched by conventional plasters. Therefore they are bestsuited for application where a good filterability is required, as it isthe case for gypsum fiberboard. One application of plasters, very oftenforgotten in the art, is its use as a binder in fiber reinforcedproducts made out of a pulp of fibers and plaster, where a considerableamount of surplus water has to be removed either by suction or bycompression filtering. The resistance to mechanical destruction duringmixing of treated plasters of the invention is unmatched by conventionalplasters. Therefore they are best suited for application where a goodfilterability is required, as it is the case for gypsum fiberboard.

1. Process for stabilizing a calcined β-hemihydrate plaster comprisingthe steps of: a) providing a heated hemihydrate plaster preferably attemperatures over 100° C.; b) feeding the heated plaster in a moisteningdevice having walls heated to at least 100° C.; c) injecting waterand/or steam into the moistening device in conditions that such not yetmoistened surfaces of the plaster are exposed to the injected waterand/or steam; d) maintaining an atmosphere in the moistening device at alevel of the dew point in the range of 75 to 99° C.; e) feeding themoistened blend into a curing device; f) maintaining an atmosphere inthe curing device above 75° C. for at least 3 minutes; g) feeding themoistened and cured blend into a drying device; and h) drying saidmoistened and cured blend.
 2. The process of claim 1, further comprisingthe steps of: i) grinding the dried product; and/or j) cooling the driedproduct, where the cooling can take place before or after the grindingstep, if any.
 3. The process of claim 2, wherein the grinding andcooling are combined into a single process step.
 4. The process of claim2, where the drying device is an indirectly heated device and theproduct temperature is 80 to 110° C.
 5. The process of claim 2, wherethe drying device is an air heated device and the product temperature is50 to 95° C.
 6. The process of claim 2, where the drying device is anindirectly heated device and the product temperature is 95 to 105° C. 7.The process of claim 2, where the drying device is an air heated deviceand the product temperature is 60 to 80° C.
 8. The process of claim 1,where the plaster of step a) has a temperature of 100 to 135° C.
 9. Theprocess of claim 1, where the walls are heated at a temperature of 100to 150° C.
 10. The process of claim 1, where water and steam areinjected by one or several two phase nozzles.
 11. The process of claim1, where water and/or steam is sprayed on a cascading plaster powder.12. The process of claim 1, where water and/or steam is sprayed on afluidized bed of plaster powder.
 13. The process of claim 1, where thefree moisture, based on the weight of the hemihydrate, is from 3 to 12%.14. The process of claim 1, where the dew point of step d) is 80 to 95°C.
 15. The process of claim 1, where the curing step f) is carried outwhile maintaining an atmosphere in the range of 75 to 99° C.
 16. Theprocess of claim 15, where the temperature of step f) is 80 to 95° C.17. The process of claim 1, where the curing time is 4 to 15 minutes.18. The process of claim 1, wherein the moistening and curing areperformed in the same apparatus.
 19. The process of claim 1, where thethus-obtained plaster is free of Anhydrite III and substantially free ofdihydrate.
 20. The process of claim 1, where the thus-obtained plasterhas a LOI of 6.2 to 8% calculated on a 100% purity.
 21. The process ofclaim 1, where the thus-obtained plaster has a d50 of 30 to 100 μm or ad50 of 20 to 30 μm or a d50 of 15 to 22 μm or a d50 of 10 to 20 μm, oris bimodal with a first peak at 3 to 10 μm and a second peak at 20 to 60μm.
 22. The process of 1, where the plaster originates from FGD gypsumor any other chemical by-product gypsum.
 23. The process of claim 1,where the free moisture, based on the weight of the hemihydrate, is from3.5 to 6%.
 24. The process of claim 1, where the curing time is 5 to 10minutes.
 25. The process of claim 1, wherein the heated hemihydrateplaster is provided at a temperature over 1000° C.
 26. The process ofclaim 1, wherein the thus-obtained plaster has a LOI of 6.2 to 7.3%,calculated on 1000% a purity.